Don't really like referring to riders as the worst, so maybe "one hit wonders" will do.
How about:
Erich Maechler - 1987 MSR
Gabriele Colombo - 1996 MSR
Vladislav Bobrik - 1994 Giro di Lombardia
Jean Marie Wampers - 1989 Paris-Roubaix
Erik Van Lancker - 1990 Liege-Bastogne-Liege
though I let Van Lancker off as I stood on the podium with him at the 1985 Milk Race (after winning a contest on kids' TV...)
How about:
Erich Maechler - 1987 MSR
Gabriele Colombo - 1996 MSR
Vladislav Bobrik - 1994 Giro di Lombardia
Jean Marie Wampers - 1989 Paris-Roubaix
Erik Van Lancker - 1990 Liege-Bastogne-Liege
though I let Van Lancker off as I stood on the podium with him at the 1985 Milk Race (after winning a contest on kids' TV...)